Spark plug



NOV. 27, 151 M, MARTY ET AL 2,576,469

In ven for: Maurice du/zs M r y Georges Henri MAN/8r a? A flarn y Patented Nov. 27,1951

SPARK PLUG Maurice Jules Marty, Paris, and Georges Henri Muller, Poissy, France Application April 29, 1949, Serial No. 90,388 In France May 3, 1948 4 Claims.

ternal combustion engines are classed, according to their thermal degree, that is to say according to their average Working temperature, into hot plugs and cold plugs.

For a particular engine, there will be selected a plug the thermal degree of which is suited to the characteristics of the engine in question. Thus, for engines with low efliciency, of which the volumetric ratio of compression is not high, hot plugs will be employed, while for engines with 'high efiiciency and high volumetric ratio it is necessary to employ cold plugs. ""As the mean temperature of the surface of 'the combustion chamber of an internal combustion engine is known to be the lower, the higher the efliciency of the engine, it may be said that the spark plug of the engine must have a mean working temperature approximating the mean temperature of the surface of the combustion chamber forming part thereof. I However, the cold plugs hitherto employed do not give entire satisfaction. Besides the fact that they are cold only because their insulations are generally disposed far from the combustion chamber of the engine and, consequently, present an insufiicient fouling surface, they give good results only at a high output of the engine, when the latter h'as'reached its mean working temperature, and they are unable to' assure good cold starts, especially in the case of very cold weather, or to stand rich mixtures required by'frequent pickups, low working conditions and operations with reduced load of the engine at the times of starting and during city driving, etc.

On the other hand, the various elementsof a plug of one of the conventional types, either hot or cold, operate at different temperatures for a given R. P. M. of the engine and are subjected to different temperature variations, for a given engine, according to the laws of variation of the R. P. M. and load of the engine.

It is therefore conceivable that none of these plugs can give satisfactory results, not only for all types of an engine, but not even for a given engine, in all its operative conditions.

The object of the present invention is to provide a spark plug which is free from said shortcoming and capable of giving best results on any type of engine and for all working conditions of the engine. This plug is remarkable for its variable thermal degree, that is to say it is hot at the time of starting the engine and for all reduced-load periods when the engine efficiency is low. while it iscold as soon as the starting of the engine is assured and for all full-load periods characterized by a high output of the engine. This result is obtained by imparting a peculiar form to that portion of the insulator adjacent the inner end of central electrode of the plug and is most likely to be either too cold at starting or too hot under high efliciency conditions.

According to the invention, the inner end of the insulator takes the form of a substantially spherical portion forming the inner tip of the insulator and an inwardly tapering neck portion uniting said substantially spherical tip portion to the body of the insulator. The neck portion intersects the substantially spherical tip portion of the insulator in a restricted section which is the smallest diameter of the neck portion and is substantially smaller in diameter than the tip portion.

7 A spark plug in accordance with the invention .is shown by way of example in the accompanying drawing which is a partial longitudinal section of the spark plug.

7 In the form shown in the drawing, the spark plug comprises a cylindrical shell portion I, provided with a ground electrode 2. The inner end portion of the shell forms a substantially cylindrical chamber 24. A central electrode 3 disposed on the longitudinal axis of the shell is surrounded by an insulator 4 which supports the central electrode 3 in the shell. The inner end of the central electrode projects beyond the insulator, permitting sparks to jump between the free inner end of the central electrode and the free end of the ground electrode 2.

At its inner end, situated in proximity to the free inner end of the central electrode 3, the insulator 4 has a spherical portion 5 forming the inner tip of the insulator and disposed in the cylindrical chamber portion 24 of the shell. An inwardly tapering neck portion 6 unites the substantially spherical tip portion 5 with the body portion of the insulator 4. The neck portion 6 is in the form of a truncated cone which at its smaller base intersects the substantially spherical tip portion 5 in a restricted section 1 which is the smallest portion of the neck portion and preferably has a diameter about one-half that of the spherical tip portion 5. The intersection of the truncated cone-like surface of the neck portion '6 with the spherical surface of the tip portion 5 provides an abrupt change in section of throttling of the insulator 4 at the point where the neck portion joins the tip. As viewed in longitudinal section in the drawing, there is a sharp sections are not desirable for the good operation This fact has been repeatedly proved exof a spark plug designed according to the present invention.

There will be now described, with reference to the drawing the operation of the plugin accord ance with the invention as a plug of variable thermal degree.

The inventors have found that the transmission of heat from end toend of a refractory piece one end of which is in a; hot zone and the other in-a cold zone'is efiectivefirst on its-surface and proceeds only thereafter through the interior of theinsulating mass. Consequently, in the case contemplated, at the time of starting the engine it is first theouter surface of the spherical tip 5 thatgrows hot, the hottest portion at this instant-being shaded, asat ll, on the verticalhalfsection of the insulator. The throttling l hinders the-superficial transmission of heat from the portion H toward the coldzone-constituted by the endof plug 18 in contact with the surrounding air and the heat-conductive metal gasket 20 in contact with the conventionally cylindrical shell I of the plug, this shell being in turn in contact with the engine cylinder-head (not shown) which is well cooled through its screwthread 12. Consequently the heat travels gradually, in the direction indicatedbyarrows 19, 2| and 22, first toward the inside of the spherical tip 1, a length of time during which this tip is still'hot, and is then rapidly carried away, asdesired, through thethrottling- 1 toward the region 6 of increasing transverse sections and thence through the gasket 20 by the'shell l and partly through the entire mass 18- of the plugsinsulator. From this time on, as thedissipation of the heat canibe effected through the whole mass ofthe insulator and shell the mean temperature of the spherical tip 5 decreases since the amount of heat initially concentrated in thesuperficial zone 11 is now distributed through the whole volume of theinsulator 4, whereforethe plug, which was hot at the time of starting, becomes-cold when the'engineis running. a It will be colder, in

its spherical tip 5 directly in contact with the I combustion chamber, the colder and the better ventilated the insulator end [8 is, that is to-say for example in an automobile, when it moves more rapidly. It will be the colder by construction, the shorter the height of the conic portion of insulator situated beyond the throttling 1 is,

that is to say the wider thecone is and the-closer the gasket is, within permissible limits of construction, to the throttling l. y In contrast, the plug the hotter, the lesser will be the ventilation of the cold zone I8 is, i.- e. in an automobile, when it is moving at low speed and with a more .reduced load, or when by construction the conic portion of insulator situated beyond the throttling 1 has a greater height. The spherical tip 5,.therefore, acts as a thermalregulator following the changes in the engine operation and more particularly changes in the cold .zone oftheplug. Moreover, when the engine has just stopped, the

dissipation of heat .does not. take "place as because of the absence, on its insulator, of fouling due to condensations likely to ground the plug, the portion thereof most unlikely to receive such fouling being precisely the spherical perimentally. It is, however, unimportant that saidvcondensations and deposits collect upon the conicp'ortion, beyond the throttling l, owing to .the fact that this conic portion is colder than the spherical tip.

Indeed, the sole fact that the spherical tip is free from condensations and deposits assures a good start of the engine.

Of course, the ratio of the volume of the chamber 24 'in the plug shell to the volume of the spherical portion 5 of insulator, such-as shown in the drawing, hasitsimportance inthematter of good operation of the plug. This ratio may be conveniently equal to three-to one, for example.

It'is now clearthat preferably a strictly-spherical tip should be employed, :since the sphere is the body presenting the largestvolume for a given surface area. This form of insulator tip isparticularly efiectiveaccordingto the invention, not only at the time of starting and stopping an engine, but also in running, since its surface has a very considerably emmisive 'powerwhereby its heating will be greatly reduced,whilepre-ignition will be avoided owing to the fact thatth'e sphere has no outstanding hot'points.

What is claimed is:

1. A spark plug comprising a shell, the free inner chamber ofwhich; is substantially cylindrical, a central electrode located along the longi tudinal axis of said cylindrical chamber, an 'insulatorsurrounding said electrode 'and'maintain ingv the same in the shell, said insulator compris ing a body portion, a substantially spherical portion'forming the inner tipofthe insulator and disposed 'in said free inner cylindrical chamber o'f-said shell and a truncated cone-likeneck portion uniting. said substantially spherical tip portion to thebody of the insulator and forming an abrupt change in section at the intersection of the smallest end of the cone-like neck portion with said spherical tip portion, and a ground electrodeffixed to the shell and cooperating with said central electrode.

2. A spark plug comprising a shell, the free inner chamber of which is substantially cylindrical, a central electrode located along the longitudinal axis of said cylindrical chamber, an in; sulator surrounding saidelec'trode and maintain; ing the samein the shell, said insulator comprise ing a body portion, a substantially spherical portion forming the inner tip .of the insulator .anddisposed in said free inner cylindrical chame portion with said sphericaltip portion, and a ground electrode fixedto the shell and ..COOpB1.at: ing with said central electrode; 7 p v 

